Laser peening process and apparatus using a liquid erosion-resistant opaque overlay coating

ABSTRACT

The invention relates to a method and apparatus for improving properties of a solid material by providing shockwaves there through. Laser shock processing is used to provide the shockwaves. The method includes applying a liquid energy-absorbing overlay, which is resistant to erosion and dissolution by the transparent water overlay and which is resistant to drying to a portion of the surface of the solid material and then applying a transparent overlay to the coated portion of the solid material. A pulse of coherent laser energy is directed to the coated portion of the solid material to create a shockwave. Advantageously, at least a portion of the unspent energy-absorbing overlay can be reused in situ at a further laser treatment location and/or recovered for later use.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to the use of coherent energy pulses, asfrom high power pulsed lasers, in the shock processing of solidmaterials, and, more particularly, to methods and apparatus forimproving properties of solid materials by providing shockwaves therein.The invention is especially useful for enhancing or creating desiredphysical properties such as high cycle fatigue life, fretting fatiguelife, hardness, strength, and fatigue strength.

2. Description of the Related Art

Old methods for the shock processing of solid materials typicallyinvolve the use of high explosive materials in contact with the solid,high explosive materials or high pressure gases used to accelerate aplate that strikes the solid to produce shockwaves therein. Such methodshave several disadvantages. For example: (a) it is difficult and costlyto shock process non-planar surfaces and complicated geometries, (b)storage and handling of the high explosive materials and high pressuregases pose a hazard, (c) the processes are difficult to automate andthus fail to meet some industrial needs and (d) high explosive materialsand high pressure gases cannot be used in extreme environments such ashigh temperatures and high vacuum.

Shot peening is another widely known and accepted process for improvingthe fatigue, hardness, and corrosion resistance properties of materialsby impact treatment of their surfaces. In shot peening, many small shotor beads are thrown at high speed against the surface of a material.However, the depth of treatment using conventional shot peening istypically only 0.004 to 0.006 inches deep. This depth is only 10-20percent as deep as achieved with laser peening, and the surfaceenhancement of material properties with shot peening is much lesseffective.

Laser shock processing with coherent radiation has several advantagesover what has been done before. For example, the source of the radiationis highly controllable and reproducible. The radiation is easily focusedon preselected surface areas and the operating conditions are easilychanged. This allows flexibility in the desired shocking pressure andcareful control over the workpiece area to be shocked. Workpiecesimmersed in hostile environments, such as high temperature and highvacuum can be shock processed. Additionally, it is easy to shock theworkpiece repetitively. This is desirable where it is possible toenhance material properties in a stepwise fashion. Shocking theworkpiece several times at low pressures can avoid gross deformation,cracking, and spallation of the workpiece while non-planar workpiecescan be shock processed without the need of elaborate and costly shockfocusing schemes.

Laser peening (hereinafter referred to as laser shock processing)utilizes two overlays: a transparent overlay (usually water) and anopaque overlay, previously an oil based or acrylic based black paint.Tapes, such as black polyvinyl chloride or polyethylene tapes, have alsobeen used successfully as the opaque overlay. During processing, a laserbeam is directed to pass through the water overlay and is absorbed bythe opaque overlay (black paint or tape), causing a rapid vaporizationof the opaque overlay surface and the generation of a high-amplitudeshockwave. The shockwave cold works the surface of the part as itpropagates into the material and creates compressive residual stresses,which provide an increase in fatigue properties of the part. A workpieceis typically processed by processing a matrix of overlapping spots thatcover the fatigue critical zone of the part.

The current laser processing of workpieces requires multiplere-applications of the opaque overlay, which require that the workpiecebe manually removed from the laser processing station and recoated afterseveral non-adjacent spots have been processed. When using paint, theold paint is sometimes removed before repainting the part or sometimesadditional paint is simply added over the old paint. The repainting canrequire upwards of 12 to 15 paint cycles. Each cycle usually requires15-20 minutes before the part can be returned to the processing station.This additional handling of the part for repainting will add as much as50% to the cost of the processing in a production environment. In asimilar manner, re-application of tape to overcome the problem of tapedamage from the laser shot during processing adds significantly to theprocessing time and cost.

Other drawbacks have been found to be associated with using paints, bothoil-based and water-based ones. These drawbacks stem from the need todry paint prior to use thereof. Such drying adds to the process cycletime and/or increases system requirements in order to promote fasterdrying times. Furthermore, in the case of water-based paints, the paintcan be rewet by the transparent overlay, such as flowing water, and maybe eroded from the surface of the part before the laser beam is appliedto the part. The removal of oil-based paints, not dissolved by a wateroverlay, must be achieved chemically and/or physically in a manner thatessentially dissolves the paint or results in the flaking thereof.Additionally, there is no potential of relocating unused dried paint, insitu, to a new location where it may be used so as to reduce the totalamount of paint to be applied. As such, recycling and/or reuse of paintsis not practical, given typical removal methods.

A method of automatically applying the overlays in sequence with thelaser system has been developed into an applicator system. Thisapplicator system has, when used with the laser peening system, reducedthe time of applying the overlay coatings and increased the throughputof the laser peen process. The opaque overlays used tend to be eroded bythe transparent overlay during processing and have resulted in thegeneration of a weaker shockwave than achieved with tape or dry paint.

What is needed in the art is a laser shock process that utilizes anopaque overlay that does not have to dry, will not be eroded by thetransparent overlay and yet allows for the generation of a strongshockwave that can be applied with the applicator system.

SUMMARY OF THE INVENTION

The present invention provides a method of laser shock processing thatcan be used in a production environment that significantly reducesprocessing time. The method includes the steps of coating the workpieceto be laser shock processed with a layer of a liquid opaque overlaycoating that is resistant to erosion or dissolution by the transparentwater overlay and that is resistant to drying. The liquid overlaycoating is applied to a small area at least about 2 to 3 times thediameter of the laser beam. A transparent overlay, such as water, isapplied forming a thin flowing layer over the previously coated portion.When the water has totally covered the coated portion, the laser isfired directly through the flowing water overlay and onto the coatedarea. The entire sequence and event timing is controlled by apreprogrammed microprocessor such as found in a personal computer. Thesequences are repeated for each spot to be processed along the workpiecesurface.

The invention comprises, in one form thereof, a method of improvingproperties of a solid material by providing shockwaves therein. Anenergy-absorbing coating that is resistant to dissolution by thetransparent water overlay and resistant to drying is applied to aportion of the surface of the solid material. A transparent overlaymaterial is then applied to the coated portion of the solid material. Apulse of coherent energy is then directed to the coated portion of thesolid material to create a shockwave.

The invention comprises, in another form thereof, a method of improvingproperties of a solid material by providing shockwaves therein. Anenergy-absorbing coating that is resistant to dissolution by thetransparent water overlay and resistant to drying is applied to aportion of the surface of the solid material. A transparent wateroverlay is then applied to the coated portion of the solid material. Apulse of coherent energy is then directed to the coated portion of thesolid material to create a shockwave. An unspent portion of theenergy-absorbing coating is displaced by the shockwave into at least oneadjacent treatment location, effectively allowing in situ reuse of thatunspent portion.

The invention comprises, in yet another form thereof, an apparatus forimproving properties of a workpiece by providing shockwaves therein. Theapparatus includes a material applicator for applying anenergy-absorbing material on to the workpiece to create a coated portionand a transparent overlay applicator for applying a liquid transparentoverlay to the workpiece over said coated portion. A laser isoperatively associated with the transparent overlay applicator toprovide a laser beam through the liquid transparent overlay to create ashockwave on the workpiece. A positioning mechanism is included toselectively position the workpiece relative to the material applicator,the transparent overlay applicator and the laser. Conversely, thepositioning mechanism may position the material applicator andtransparent overlay applicator correctly over the spot on the workpieceto be treated while it is in position in the laser beam path. A controlunit is operatively associated with each of the applicators, laser, andpositioning mechanism, to control the operation and timing of each ofthe applicators, laser, and the selective operation of the positioningmechanism.

The invention comprises, in yet a further form thereof, anenergy-absorbing overlay for use in conjunction with a laser-inducedshock process. The energy-absorbing overlay includes a base material,which is resistant to erosion or dissolution by the transparent wateroverlay and resistant to drying, and at least one energy-absorbingparticulate dispersed within the base material. Advantageously, the basematerial is an oil, and the particulate material is colloidal graphiteand/or fine black iron oxide (Fe₂O₃) (Fe ₃ O ₄).

An advantage of the present invention is that the method allows the useof energy-absorbing coating that is viscous, adherent, and resistant todrying and that lends itself to reuse/recycling. Prior laser paintingprocesses utilized oil, water or acrylic based paints that needed to bedried prior to use and that did not lend themselves to reuse and/orrecycling.

Another advantage of the present invention is that the energy-absorbingcoating that has an appropriate viscosity that allows it to both conformto a workpiece shape under substantially static conditions and bereadily displaced under sufficiently dynamic conditions. Alternately,the energy-absorbing material can be chosen so as to be self-limitingwith respect to its thickness, thereby improving thickness uniformityand reducing the need for thickness control measures.

Yet another advantage of the present invention is that since theenergy-absorbing coating (both the viscous and self-limiting thicknessversions) is resistant to drying and is typically oil based (i.e.,lubricant as base), such a coating material will not tend to stop up anapplicator, either during use or after an extended period of stagnationtherewith in. The use of extremely fine or colloidal particulateprevents clogging of the applicator and prevents problems with settlingduring storage or processing. The non-drying feature permits thepre-application of the energy-absorbing coating. This pre-applicationcan reduce the time and amount of spraying needed to be done in theprocessing cell and can limit the opportunity of applying a laser pulseto an uncoated area.

A further advantage of the present invention is the utilization of aflowing, transparent water overlay for processing of the workpiecesurface. The use of the flowing water covers the coated area uniformlywhile additionally ensuring that any heat possibly transferred to theworkpiece by the process will be removed.

Yet a further advantage of the present invention is that the processeliminates the need to move the workpiece from workstation toworkstation as was previously accomplished and necessitated. The lasershock processing system now can be adapted to manufacturing processworkloads and requirements.

A yet additional advantage of the present invention is that theapplicator(s) in the coating system can be fitted with integralprotectors that can preclude unwanted spraying of selected workpieceportions and/or selected process apparatus components, thereby botheliminating the need to manually mask such selected portions/componentsand reducing clean-up time.

BRIEF DESCRIPTION OF THE DRAWINGS

The above-mentioned and other features and advantages of this invention,and the manner of attaining them, will become more apparent and theinvention will be better understood by reference to the followingdescription of an embodiment of the invention taken in conjunction withthe accompanying drawings, wherein:

FIG. 1 is a flow chart of the method of the present invention

FIG 2 is a diagrammatic view of one embodiment of the present invention;and

FIG 3 is a diagrammatic view of another form of the present inventionutilizing a rotatable carousel to index a workpiece.

Corresponding reference characters indicate corresponding partsthroughout the several views. The exemplification set out hereinillustrates one preferred embodiment of the invention, in one form, andsuch exemplification is not to be construed as limiting the scope of theinvention in any manner.

DETAILED DESCRIPTION OF THE INVENTION

The improvements in fatigue life produced by laser shock processing arethe results of residual compressive stresses developed in the irradiatedsurface that retard fatigue crack initiation and/or slow the crackpropagation rate. A crack front is the leading edge of a crack as itpropagates through a solid material. Changes in the shape of a crackfront and slowing of the crack growth rate when the crack frontencounters the laser shock zone in a laser shock processing conditionhave been shown.

Laser shock processing is an effective method of increasing fatigue lifein metals by treating fatigue critical regions. As to what effect thetensile residual stresses surrounding the laser shocked region wouldhave on crack initiation, a previous study is described in “Shockwavesand High Strained Rate Phenomena in Metals” by A. H. Clauer, J. H.Holbrook and B. P. Fairand, ed. by M. S. Meyers and L. E. Murr, PlenumPress, New York (1981), pp. 675-702. Described in the above referenceare the effects of laser shock processing on fatigue of welded aluminumspecimens that hod used a series of overlapping spots to cover the weldand heat-affected zones. Significant increases in fatigue life wereobserved for these specimens indicating that overlapping the spots didnot significantly reduce the effects of laser shocking. This issupported by results on a low carbon steel that showed no change in thecompressive residual stress profile across the edge of a spot in aspot-overlap region.

For a more thorough background in the prior history of laser shockprocessing and that of high power processing of engineered materials,reference can be made to U.S. Pat. No. 5,131,957, such patent explicitlyhereby incorporated by reference. This patent also shows a type of laserand laser circuit adaptable for use with the present invention. Anothertype of laser adaptable for use with the invention is that of a Nd:GlassLaser manufactured by LSP Technologies, Inc. of Dublin, Ohio.

Overlays are applied to the surface of the target workpiece being lasershock processed. These overlay materials may be of two types, onetransparent to laser radiation and the other opaque to laser radiation.They may be used either alone or in combination with each other, but itis preferred that they be used in combination with the opaque overlayadjacent the workpiece, and the outer transparent overlay being adjacentthe opaque overlay.

The transparent overlay material should be substantially transparent tothe radiation. Useful transparent overlay materials include water,water-based solutions, other non-corrosive liquids, glass, quartz,sodium silicate, fused silica, potassium chloride, sodium chloride,polyethylene, fluoroplastics, nitrocellulose, and mixtures thereof.Fluoroplastics, as they are known by ASTM nomenclature, are parallinichydrocarbon polymers in which all or part of each hydrogen atom has beenreplaced with a fluorine atom. Another halogen, chlorine, can also bepart of the structure of a fluoroplastic. By order of decreasingfluorine substitution and increasing process ability, these materialsinclude polyletrafluoroethylene (PTFE); fluorinated ethylene propylene(FEP): the chlorotrifluorethylenes (CTFE); and polyvinylidine fluoride(PVF.sub.2). Also available is a variety of copolymers of bothhalogenated and fluorinated hydrocarbons, including fluorinatedelastomers. Additionally, the transparent overlay could be a gel or astrip of tape comprised of one or more of the above materials.

Where desired, the opaque overlay material may be substantially opaqueto the radiation. Useful opaque overlay materials of the presentinvention include an energy-absorbing dispersant and a base material Thedispersant may, for example, be graphite, carbon black, black iron oxide(Fe₂O₃) (Fe ₃ O ₄), and/or mixtures of these materials. The basematerial for such opaque overlay materials is advantageously an oil(e.g., mineral, vegetable, or petroleum-derived) which contributes tothe viscosity control, adherence, and drying resistance needed by theopaque overlay material of the present invention. The mixture of thedispersant and base material is colloidal in nature, in the sense thatthe dispersant is too fine to be filtered readily from the base materialand is resistant to settling. A typical overlay is about 10 to 20,000micrometers (μm) thick. In an advantageous embodiment of the invention,an oil-and-graphite mixture is used to give superior results both interms of energy absorption during shock processing and reuse thereafter.

Referring now to the drawings and particularly to FIG 2, there is showna preferred embodiment 10 of the present invention including a targetchamber 12 in which the laser shock process takes place. The targetchamber 12 includes an opening 14 for a laser beam 16 created by laser18, a source of coherent energy. Laser 18, by way of example, may be acommercially available high power pulse laser system capable ofdelivering more than approximately 40 joules in 5 to 100 nanoseconds.The laser pulse length and focus of the laser beam may be adjusted asknown in the art.

Shown in FIG 2, a workpiece 20 is held in position within target chamber12 by means of a positioning mechanism 22. Positioning mechanism 22 maybe of the type of a robotically controlled arm or other apparatus toprecisely position workpiece 20 relative to the operational elements oflaser shock system 10.

System 10 includes a material applicator 24 for applying anenergy-absorbing material onto workpiece 20 to create a coated portion.Material applicator 24 may be that of a solenoid operated paintingstation or other construction such as a jet spray or aerosol unit toprovide a small coated area onto workpiece 20. The material utilized bymaterial applicator 24 is preferably an energy-absorbing material thatis resistant to erosion or dissolution by the water overlay andresistant to drying, or an energy-absorbing material that isself-limiting in thickness yet adherent and resistant to drying. Inaddition, the opaque overlay may advantageously be resistant to ignitionby the plasma plume. Each such material advantageously is composed of anoil and graphite mixture. Various features of the oil/graphite mixturesuch as the graphite particle size, graphite concentration, oilcomposition, and additive usage con be adjusted to produce the desiredset of material characteristics. Alternatively, other types of opaquecoatings may be used that incorporate those materials discussed above.

With respect to the energy-absorbing material that exhibitsself-limiting coating thickness, the oil-based nature of the colloidalgraphite coating exhibits a tendency to limit the coating build-upduring spray application. The coating wets the surface of parts welland, due to its relatively low viscosity, it spreads out ratheruniformly when deposited on a given surface. Like the viscous version ofthe energy-absorbing material, the self-limiting coating thicknesscomposition does not dry during application, so it is less susceptibleto clumping, clogging, or similar spray problems encountered withwater-based paints that have been previously evaluated.

As the oil-based coating of this embodiment is sprayed, it readilydisplaces laterally, and the coating thickness at the spot where thelaser beam will be applied reaches a self-limiting maximum. Overspray ofthe coating simply displaces sideways and does not affect the laserpeening operation. As such, the self-limiting nature of this coatingembodiment provides improved uniformity to the coating thickness, whichtheoretically should improve the uniformity of the shockwave created atthe surface during laser peening. Further, the spreading characteristicof the coating makes control of the coating thickness much less criticalthan when using coatings, which are susceptible to oversprayirregularities.

The non-drying nature of the oil-based colloidal graphite coating (boththe viscous and self-limiting thickness compositions) offers anopportunity to pre-coat or pre-spray the part before laser peening. Insome instances, this may reduce the amount of overlay coating that mustbe applied by material applicator 24 (which may be incorporated into aRapidCoater™ system (not shown).

Using this approach, the part is pre-coated, and material applicator 24can be used just to “touch up” the spot to be laser peened. Thisprocedure can reduce the coating spray cycle time in the processing celland potentially increase productivity in situations where the coatingapplication cycle time is the time-limiting parameter. This methodologycan also be used to minimize the amount of spraying in the work cell,which can reduce overspray on the workpiece and potential contaminationof objects associated with the process system. Furthermore, pre-coatingthe part will also lessen the chances of applying a laser beam pulse toan uncoated area, which can result in melting, “burning”, and/or“staining” of the surface.

System 10 further includes a transparent overlay applicator 26 thatapplies a fluid or liquid transparent overlay to workpiece 10 over theportion coated by material applicator 24. Transparent overlay materialshould be substantially transparent to the radiation as discussed above,water being the preferred overlay material.

As shown in FIG 2, both material applicator 24 and transparent overlaymaterial applicator 26 are shown directly located within target chamber12. In a production operation environment, only the necessary operativeportions need be located through and within target chamber 12 such asthe portion through which the materials actually flow through a flowhead. The supply tanks for the transparent overlay materials and otherenergy-absorbing materials may be located outside of target chamber 12.

Both material applicator 24 and transparent overlay material applicator26 can each be fitted with an integral protector 37 schematicallyindicated in FIG. 3. Each protector 37 extends beyond a respective spraynozzle 39 of the particular one of material applicator 24 and overlaymaterial applicator 26. Such protectors 37 are attached to therobotically-positionable coating system fixturing (not labeled). Thus,the shielding protector 37 accompanies a given nozzle 39, and manualapplication of protectors to the workpiece and/or specific portions ofthe process apparatus can be obviated. Protectors 37 may be made of anysuitable material, such as polyethylene, Lexan™, or other plastics.Metal protectors can be used, but plastics are typically preferred tominimize the potential for scratching parts during processing ormovement of applicators 24 and 26 and such or other portions of theRapidCoater™ System.

A control unit, such as controller 28, is operatively associated witheach of the material applicator 24, transparent overlay materialapplicator 26, laser 18 and positioning mechanism 22. Controller 28controls the operation and timing of each of the applicators 24, 26,laser 18 and selective operation of positioning mechanism 22 to ensureproper sequence and timing of system 10. Shown in FIG. 2, controller 28is connected to laser 18, positioning mechanism 22, material applicator24 and transparent overlay material applicator 26 via control lines 30,32, 34 and 36, respectively. Controller 28, in one embodiment, may be aprogrammed personal computer or microprocessor.

In operation, controller 28 controls operation of system 10 onceinitiated. As shown in FIG. 1, the method of the invention is thatfirst, workpiece 20 is located (38) particularly within targetingchamber 12 by positioning mechanism 22. Controller 28 activates materialapplicator 24 to apply a laser energy-absorbing coating 40 onto aparticular location of workpiece 20 to be laser shock processed. Thenext step of the process is that controller 28 causes transparentoverlay material applicator 26 to apply transparent overlay 42 to thepreviously coated portion of workpiece 20. At this point, laser 18 isimmediately fired by controller 28 to initiate a laser beam 16 to impactthe coated portion 44. Preferably, the time between applying thetransparent water overlay and the step of directing the laser energypulse is approximately 0.1 to 3.0 seconds. By directing this pulse ofcoherent energy to the coated portion, a shockwave is created. As theplasma expands from the impact area, it creates a compressionalshockwave passing through and against workpiece 20.

As part of the laser peening system to be employed, it is advantageousto provide a detection/monitoring unit 45 which can be used to ensurethat the overlay coatings for each spot during each cycle have beenappropriately applied and that the laser beam has been applied to thepart before moving on to the next spot. Detection/monitoring unit 45 maybe any one of a mass/flow meter, a video monitoring unit, a plasmamonitor, and an acoustic monitor. Mass/flow meters may be provided onthe coating and water supply lines to applicators 24 and 26,respectively. Digital outputs from the meters may be monitored withcontroller 28. In the basic form thereof, a pulse from the meters can beused to verify application of an overlay through a system of spraynozzles 39 or to signal an alarm condition if no flow is registered.With more sophisticated meters, a quantitative measure of the amount ofthe overlay coating can may be registered and recorded. Other sensorsfor detecting the presence of the overlay coating may also be used, suchas ultrasonic flow/motion detectors or interrupted optical signals.

Another alternative for detection/monitoring unit 45 is a video monitor.With such a video monitor, the spray pulse from the system nozzles 39may be observed with video monitors focused at locations showing thenozzles 39 positioned to spray the part. Image analysis can be used inreal time during the spray cycle to verify the application and the spotlocation. In the case of graphite, oil-based coatings, the spray patternmay be difficult to observe with a video monitor because of the darkcolors of the coating and the substrate.

As a yet further alternative for detection/monitoring unit 45, a plasmamonitor can be used which can verify application of the coatingoverlays. For example, the flash of the plasma during the application ofthe laser beam has a characteristic signature that can be analyzed inreal time. This characteristic signature can be used to determine ifwater overlay coating was in place as the laser pulse was applied.

As a yet further alternative, one or more acoustic monitors can be usedfor detection/monitoring unit 45. The presence of the overlay coatingscan be verified using acoustic monitors such as a pinducer. Duringproper processing, the application of the laser pulse produces a loudsnap (typically >130 dB). If the overlay coatings are not present or thelaser pulse misses the part, the acoustic signature is not present, isaltered, or is greatly diminished. Thus, a minimum sound loudnessthreshold can be monitored, recorded, and used as a means of indicatingthat each spot was processed properly with the overlay coatings inplace.

During plasma formation due to laser impact, a first portion of thelaser energy-absorbing coated portion 44 is sacrificed, e.g., convertedto plasma or driven off workpiece 20. A second, unsacrificed portionthereof is prevented from escaping by transparent overlay 42 but, due toits characteristic viscosity, instead is fluidly displaced laterallyalong workpiece 20 away from the impact/impingement point of laser beam16. An advantageous by-product of this displacement is that anunspent/unsacrificed portion of the energy-absorbing overlay 44 can befluidly displaced into one or more adjacent potential treatmentlocations on workpiece 20 or recovered for reuse (i.e., Reuse/RecycleStep 46). That the improved oil-based energy-absorbing overlay 44spreads well, is readily fluidly displaced, and is self-limiting inthickness can effectively reduce or eliminate the need for further laserenergy-absorbing coating 40 to be deposited onto such a potentialtreatment location. A detection/monitoring unit 45 (which may be adirect (i.e., contact) or indirect (i.e., photo) unit as appropriate) isprovided for measuring the thickness of the coated portion 44 at atreatment location in order to determine if more laser energy-absorbingcoating 40 (i.e., a touch-up application thereof) need be appliedthereto prior to firing of laser 18. Using this approach of maximizingthe applied energy-absorbing overlay 44 can result in reduced coatingcycle times and thereby result in increased productivity in cases wherecoating application cycle times is the rate-limiting portion of thelaser peening process.

With the use of the improved oil-based overlay coating compositions ofthe present invention, a step of cleaning a laser-peened spot with anair/water blast to remove the paint and to prepare for the next laserpeening system cycle may be performed or omitted, as desired. Because ofthe improved nature of the overlay coatings of the present invention, itis not always necessary to remove the coating between overlay spraycycles. This is especially true when the self-limiting thicknesscomposition is employed. A touch-up of the overlay coating can beadequate to replenish the overlay coating layer sufficiently to promoteburning and to generate the shockwave. Using this approach, the overallcycle time for the laser peening process can be reduced, andproductivity can be increased in cases where the coating cycle time isthe time-limiting process step during laser peening.

In the case where the overlay coating is not removed during processing,the part may be cleaned after laser peening has been completed usingvarious spray cleaning techniques such as a high-pressure cleaningsystem or cleaning systems analogous to dishwasher cleaning. Removal ofthe oil-based overlay coating may be facilitated with the use of hotwater and a detergent (preferably non-foaming). The removed coating isadvantageously immiscible with water and may be skimmed or separatedreadily from the waste water for disposal or recycling.

If it is desired to use the RapidCoater™ System with thegraphite/oil-based overlay and to remove the coating at each spot duringa given system cycle, the air/water blast portion of the cycle may beused. It is advantageous to use hot water and a non-foaming detergent tofacilitate the removal of the overlay coating. Many suitable detergentsare available, one such non-foaming detergent being AP-12 available fromEMD Chemicals, Inc.

The above-described process or portions of the process are repeated toshock process the desired surface area of workpiece 20. Depending uponthe energy levels and the amount of laser shocking desired on workpiece20, controller 28 may position or re-index workpiece 20 into anotherposition using positioning mechanism 22, so that system 10 may applycoatings to and laser beam 12 16 may impact a different portion ofworkpiece 20, which may overlap the previously impact impacted area.

It may be advantageous in a production environment to separate thecoating operation from the lasing operation. FIG. 3 discloses anotherway to reposition and re-index a workpiece 20 using a selectivelyrotatable carousel 50. Positioning mechanism 22 is located on carousel50 for rotation therewith. Controller 28 will control both the operationof positioning mechanism 22 and the location of the mechanism byselectively rotating carousel 50 to position a workpiece adjacent eithermaterial applicator 24 or transparent overlay material applicator 26. Ifnecessary, additional workstations may be utilized but these may reducethe process efficiency.

In one form of the invention, it may be possible such that all the stepsof the process shown in FIG 1 may be accomplished without moving theworkpiece 20. Workpiece 20 may be indexed if necessary to cover largeareas by the process. By not moving workpiece 20, additionalmanufacturing efficiencies are produced while additionally reducing thetime between steps. The limiting factor in the process currently is thecycle time of laser 18. Laser 18 may be fired about every 0.5 seconds to10 seconds. This minimum amount of time is required by laser 18 torecharge while permitting selected other steps of the process shown inFIG. 1 to be accomplished.

In another form of the invention, controller 28 can be a smart controlunit advantageously interfaced with the RapidCoater™ System. Thisinterface promotes the parameters and timing of the coating system to beset remotely by the operator. Potentially, the settings can be variedspot-by-spot using a program sequence. Movement of the part and thelaser peening system robots (not labeled) can be coordinated for optimalprocessing. Detection/monitoring unit(s) 45 used to validate theapplication of the overlay coatings and/or the laser spot can beinterfaced to controller 28, which is specifically designed to be asmart controller as part of this embodiment. Using this approach,processing can be verified for each spot before proceeding to the next.This verification system is much more efficient than re-positioning torework various spots after the primary processing is completed.

In alternative embodiments, the application of transparent overlaymaterial (shown in step 42) may comprise of applying the transparentoverlay material continuously before and during the directing of thelaser energy pulse.

Depending upon the workpiece material, many parameters of the presentinvention may be selected to control the shock process. For example, theoperator controller may select a particular laser pulse energy, laserpulse time, number of laser pulses, focal lens, working distance,thickness of both the energy-absorbing coating and transparent overlayto control the laser shock process. More particularly, laser pulseenergy and laser pulse width directly affect this cycle. The amount ofenergy placed on the surface of the workpiece and number of laser pulsesaffects the depth of each shock and the speed of the shocking process.It has been found that the energy of the laser pulse, as well as otherparameters should be controlled in order to prevent surface melting ofthe workpiece.

While this invention has been described as having a preferred design,the present invention can be further modified within the spirit andscope of this disclosure. This application is therefore intended tocover any variations, uses, or adaptations of the invention using itsgeneral principles. Further, this application is intended to cover suchdepartures from the present disclosure as come within known or customarypractice in the art to which this invention pertains and which fallwithin the limits of the appended claims.

What is claimed is:
 1. A method of for treating a workpiece by applyingshockwaves thereto , comprising the steps of : applying an a liquidenergy-absorbing overlay to a portion of a surface of the workpiece,said energy-absorbing overlay being composed of a liquid materialresistant to dissolution by the transparent water overlay and resistantto drying the liquid energy-absorbing overlay comprising a liquidmaterial resistant to drying, and applying a transparent overlay uponthe liquid energy-absorbing overlay, wherein the liquid energy-absorbingoverlay is comprised of a liquid material resistant to dissolution bythe transparent overlay; applying a transparent overlay upon saidenergy-absorbing overlay; and directing a pulse of coherent energy tosaid the liquid energy-absorbing overlay, said the pulse of coherentenergy being absorbed at least in part by said liquid material andcausing a portion of said the liquid energy-absorbing overlay tovaporize and thereby generate ; and generating at least one shockwavefor transmission to the workpiece.
 2. The method of claim 1, whereinsaid the liquid erosion-resistant and drying-resistant material has acombined viscosity and level of adherence such that said the liquidenergy-absorbing overlay made thereof tends to conform and adhereconforms and adheres to the workpiece under substantially staticconditions, yet is capable of fluid displacement when subjected to theat least one shockwave.
 3. The method of claim 1, wherein said theliquid erosion-resistant and drying-resistant material is comprises acolloidal substance having comprising at least one energy-absorbingparticulate dispersed therein .
 4. The method of claim 3, wherein saidthe colloidal substance is a mixture comprised of an oil and graphite.5. The method of claim 3 wherein said the colloidal substance is amixture comprised of an oil and black iron oxide (Fe₂O₃) .
 6. The methodof claim 3, wherein said the colloidal substance is a mixture comprisedof an oil, colloidal and at least one of graphite, carbon black, andblack iron oxide (Fe₂O₃) .
 7. The method of claim 1, wherein saidenergy-absorbing overlay has a viscosity of a magnitude that permitssaid energy-absorbing overlay to conform with a surface of the workpieceunder substantially static conditions and yet to be fluidly displacedwhen subjected to sufficiently dynamic conditions.
 8. The method ofclaim 7, wherein the sufficiently dynamic conditions occur during atleast one of said applying an energy-absorbing overlay step and saiddirecting a pulse step.
 9. The method of claim 1, wherein said theliquid energy-absorbing overlay includes comprises at least a firstoverlay portion and a second overlay portion, said first overlay portionbeing sacrificed upon impact of the pulse of coherent energy, said thesecond overlay portion being reusable for a subsequent shockwavecreation generation.
 10. The method of claim 9, wherein the secondoverlay portion is fluidly displaced laterally along the workpiecesurface, away from an impingement point of the pulse of coherent energy,an amount of the second overlay portion being displaced into an otherproximate treatment location upon the workpiece.
 11. The method of claim10, further comprising the further steps of : applying a an additionaltransparent overlay on upon the amount of the second overlay portiondisplaced into the other proximate treatment location; and directing asecond pulse of coherent energy through the transparent overlay to theamount of the second overlay portion displaced into the other proximatelocation to effect a shockwave formation thereat , generating at leastone shockwave for transmission to the workpiece.
 12. The method of claim11 10, further comprising the steps of : monitoring the amount of thesecond overlay portion displaced into the other proximate treatmentlocation, said monitoring thereof being performed prior to the step ofapplying the transparent overlay thereto ; and adjusting a totalthickness of the energy-absorbing overlay existing at the otherproximate treatment location to thereby conform with a desired thicknesstherefor .
 13. The method of claim 1, further comprising the step ofreclaiming any remaining amount of said the energy-absorbing overlay.14. The method of claim 1, wherein the coherent energy is in a form oflaser energy.
 15. A method of for treating a workpiece by applyingshockwaves thereto , comprising the steps of : applying an a liquidenergy-absorbing overlay to a portion of a surface of the workpiece,said the liquid energy-absorbing overlay being composed comprised of anadherent, uniformly spreading liquid material, said adherent, uniformlyspreading liquid material being that is resistant to drying; applying atransparent overlay upon said the liquid energy-absorbing overlay; anddirecting a pulse of coherent energy to said the liquid energy-absorbingoverlay, said the pulse of coherent energy being absorbed at least inpart by said the liquid material and causing a portion of saidenergy-absorbing overlay to vaporize and thereby generate , generatingat least one shockwave for transmission to the workpiece.
 16. The methodof claim 15, wherein said the adherent, uniformly spreading liquidmaterial displaces easily enough laterally when sprayed so as to therebyreach a coating thickness having a self-limiting maximum.
 17. The methodof claim 15, further comprising the step of : one of pre-coating andpre-spraying the workpiece with another a second adherent, uniformlyspreading liquid material prior to said step of applying said the liquidenergy-absorbing overlay, said another the second adherent, uniformlyspreading liquid material being resistant to drying.
 18. The method ofclaim 17, wherein said step of applying said energy-absorbing overlayincludes supplying said adherent, uniformly spreading liquid material atlocations where it is needed and one of lacking and supplied at aninsufficient thickness.
 19. The method of claim 15, further comprisingthe step of: cleaning the workpiece after the treating of the workpieceby applying shockwaves thereto, said step of the cleaning the workpiecebeing by a spray cleaning technique.
 20. The method of claim 15, whereina plurality of spots are treated during the treating of the workpiece ,said the liquid energy-absorbing overlay being applied to each of saidspots spot individually, said the method further comprising the step of: removing said the liquid energy-absorbing overlay from each said spotafter performing the step of directing the pulse of said coherent energyupon to each said spot.
 21. The method of claim 15, further comprisingthe step of : using an automated means for ensuringdetecting at leastone of that a correctsufficient amount of saidthe liquidenergy-absorbing overlay has been applied at a given treatment spot andthat the laser beampulse of coherent energy has been applieddirected atthe given treatment spot prior to a next treatment step being performed.
 22. The method of claim 21, wherein said automated means is configuredfor the detecting includes measuring an applied amount of said theliquid energy-absorbing overlay, said automated means being one of amass/flow meter, a video monitor, a plasma monitor, and an acousticmonitor .
 23. The method of claim 15, wherein at least one first spraynozzle is used for the applying said the liquid energy-absorbing overlaycomprises applying with at least one first spray nozzle, at least onesecond spray nozzle being used for the applying said the transparentoverlay comprises applying with at least one second spray nozzle, eachsaid first spray nozzle and each said second spray nozzle having aprotector fitted associated therewith, each said the protector beingconfigured for shielding a segment of the workpiece from potentialdamage from a coating material being ejected through a given said spraynozzle .
 24. The method of claim 15, wherein said drying-resistant theadherent, uniformly spreading liquid material is comprises a colloidalsubstance having comprising at least one energy-absorbing particulatedispersed therein .
 25. The method of claim 24 wherein said colloidalsubstance is a mixture of oil and black iron oxide (Fe₂O₃).
 26. Themethod of claim 24 wherein said colloidal substance is a mixture of oil,colloidal graphite and black iron oxide (Fe₂O₃).
 27. The method of claim15, further comprising the step of reclaiming any remaining amount ofsaid energy-absorbing overlay.
 28. The method of claim 21, wherein thedetecting comprises detecting with at least one of a mass/flow meter, avideo monitor, a plasma monitor, and an acoustic monitor, an ultrasonicflow detector, an ultrasonic motion detector, and interrupted opticalsignals.
 29. A method for treating a workpiece by applying shockwavesthereto, comprising: applying an energy-absorbing overlay to theworkpiece, the energy-absorbing overlay being in liquid form, beingresistant to drying, and being comprised of at least one of carbonblack, graphite, and black iron oxide; and directing a pulse of coherentenergy to the energy-absorbing overlay while the energy-absorbingoverlay is in the liquid form, wherein the pulse of coherent energy isabsorbed at least in part by the energy-absorbing overlay.
 30. A methodfor treating a workpiece by applying shockwaves thereto, comprising:applying an energy-absorbing overlay to the workpiece, theenergy-absorbing overlay being in liquid form; and directing a pulse ofcoherent energy to the energy-absorbing overlay while theenergy-absorbing overlay is in the liquid form, wherein the pulse ofcoherent energy is absorbed at least in part by the energy-absorbingoverlay, and wherein the energy-absorbing overlay is resistant todrying.
 31. A method for treating a workpiece by applying shockwavesthereto, comprising: applying an energy-absorbing overlay to theworkpiece, the energy-absorbing overlay being in liquid form andresistant to drying; directing a pulse of coherent energy to theenergy-absorbing overlay while the energy-absorbing overlay is in theliquid form, wherein the pulse of coherent energy is absorbed at leastin part by the energy-absorbing overlay; and applying a transparentoverlay upon the energy-absorbing overlay while the energy-absorbingoverlay is in the liquid form, wherein the energy-absorbing overlay isresistant to erosion by the transparent overlay.